diff -up libsoup-2.56.0/libsoup/soup-session.c.crash-under-soup_socket_new libsoup-2.56.0/libsoup/soup-session.c
--- libsoup-2.56.0/libsoup/soup-session.c.crash-under-soup_socket_new	2016-09-16 17:14:27.000000000 +0200
+++ libsoup-2.56.0/libsoup/soup-session.c	2017-11-15 17:20:52.660392432 +0100
@@ -369,6 +369,7 @@ soup_session_finalize (GObject *object)
 	G_OBJECT_CLASS (soup_session_parent_class)->finalize (object);
 }
 
+/* requires conn_lock */
 static void
 ensure_socket_props (SoupSession *session)
 {
@@ -784,11 +785,13 @@ soup_session_set_property (GObject *obje
 		break;
 	}
 
+	g_mutex_lock (&priv->conn_lock);
 	if (priv->socket_props && socket_props_changed) {
 		soup_socket_properties_unref (priv->socket_props);
 		priv->socket_props = NULL;
 		ensure_socket_props (session);
 	}
+	g_mutex_unlock (&priv->conn_lock);
 }
 
 static void
@@ -808,7 +811,9 @@ soup_session_get_property (GObject *obje
 		g_value_set_boxed (value, priv->proxy_uri);
 		break;
 	case PROP_PROXY_RESOLVER:
+		g_mutex_lock (&priv->conn_lock);
 		ensure_socket_props (session);
+		g_mutex_unlock (&priv->conn_lock);
 		g_value_set_object (value, priv->proxy_resolver);
 		break;
 	case PROP_MAX_CONNS:
@@ -829,12 +834,16 @@ soup_session_get_property (GObject *obje
 		break;
 	case PROP_SSL_USE_SYSTEM_CA_FILE:
 		tlsdb = g_tls_backend_get_default_database (g_tls_backend_get_default ());
+		g_mutex_lock (&priv->conn_lock);
 		ensure_socket_props (session);
+		g_mutex_unlock (&priv->conn_lock);
 		g_value_set_boolean (value, priv->tlsdb == tlsdb);
 		g_clear_object (&tlsdb);
 		break;
 	case PROP_TLS_DATABASE:
+		g_mutex_lock (&priv->conn_lock);
 		ensure_socket_props (session);
+		g_mutex_unlock (&priv->conn_lock);
 		g_value_set_object (value, priv->tlsdb);
 		break;
 	case PROP_TLS_INTERACTION: